Unraveling CO2-rich fluids and daughter minerals recorded in fluid and mineral inclusions from the Martin Vaz archipelago, Vitória-Trindade Ridge (VTR), South Atlantic

Abstract

Martin Vaz Archipelago is a group of four islands formed by a succession of melanephelinite-phonolite rocks with Plio-Pleistocene ages, representing one of the latest volcanic manifestations of the Vitória-Trindade Ridge, in the South Atlantic Ocean, Brazil. This paper presents for the first-time fluid and mineral inclusions data, based on petrography and Raman spectroscopy. Olivine phenocrysts and macro phenocrysts from Atobá Formation melanephelinite host several amounts of CO2-rich fluid inclusions, although the post-entrapment process such as brittle failure, and reactions between fluids and the host mineral played a major role in these fluid inclusions evolution. Mg-rich carbonates (e.g., magnesite and dolomite) were identified as secondary minerals in the fluid inclusions walls, probably derived from these reactions. Dolomite secondary minerals formation could be related to carbonatitic fluids or carbonate-rich fluids that can be generated by a deep carbon mantle outgassing. Serpentine (antigorite) and Ca–Ti-perovskites were also found as mineral inclusions in olivine phenocrysts. Serpentine is evidence of great water activity suggesting that fluids involved in Martin Vaz genesis could have had at least during one stage of its evolution an aqueous or even an aqueous-carbonic composition. Ca–Ti-perovskite inclusions in mantle minerals have been related to eclogite-derived carbonatite metasomatic melts or to the serpentinization process in mantellic rocks, ruled by water remobilization of CaO and TiO2. In the phonolite rocks from Atobá Formation (João da Nova Member), hydroxyfluorapatite was recognized as hosting carbonate-rich melt inclusions. These apatites with incorporated volatile phases (e.g., fluorine and OH groups), and carbonate-rich melt inclusions, together with the presence of SO3-rich nosean phenocrysts are evidence of a carbonate-fluorine-sulfate brine that could be formed in the latest stages of magmatic differentiation of Martin Vaz rocks.